The present invention relates to a method of and a device for effecting electroslag remelting processes and more particularly to a method of and device for realizing an electroslag remelting of consumable metal electrodes, for casting metal ingots by the electroslag remelting technique and electroslag welding of metals and their alloys, such as, various steel grades, brasses, etc.
At present known in the art is a large number of methods for carrying into effect electroslag remelting processes. According to one of such methods for effecting the electroslag remelting of metal in a cooled mold, ingots in remelted metal are produced from metal consumable electrodes melted in a slag bath under the effect of heat given up in molten slag by the passage of electric current therethrough. Usually a known electroslag remelting plant comprises a baseplate mounting a cooled mold in which a slag bath is established and a consumable electrode is arranged movably with respect to the mold, the bottom end of said electrode being dipped into the slag bath. Under the effect of electric current flowing from a current source through the electrode, slag bath and the baseplate the electrode immersed into the slag bath fuses and remelts into an ingot.
The main disadvantages of this method and plant consist in low electrical energy efficiency and in the oxidation of easily-oxidizable elements contained in the metal.
In casting ingots by using the known method of electroslag remelting molten metal is poured into a mold filled with molten slag. According to the above method, at first solid slag is charged into the mold. After that electric current is passed through an electrode, slag bath and baseplate, said current heating and melting the slag, whereupon molten metal in a fluid state is poured into the mold where it is heated during solidification due to the passage of electric current through the electrode, molten slag and molten metal.
The above method of casting ingots by electroslag remelting is realized in a plant, comprising a mold which accommodates an electrode mounted movably with respect to the mold and adapted for melting slag contained in the mold and for subsequent heating of metal being cast.
One of the main disadvantages of said method and plant resides in high specific energy consumption.
Known also is a method of electroslag welding in which in a space bounded by the edges of articles being welded and molding devices a molten slag bath is set up into which a metal electrode is dipped and through which electric current is passed. Flowing through the electrode, slag bath and base metal the current heats molten metal and the slag bath and maintains their high temperature and electrical conductivity. The heated molten slag melts the electrode introduced thereinto and the edges of article elements being welded. The metal obtained when the edges of the article elements being welded are being melted, as well as molten metal formed by the melting electrode flow to the bottom of the slag bath establishing a metal pool which, on being solidified, molds the weld interconnecting the edges of the articles being welded.
The disadvantages of the known method of electroslag welding include low energy efficiency, oxidation of easily-oxidizable elements contained in metal and the impossibility of adjusting edges and fusion of parts being welded.
Thus, all the above-specified methods and plants suffer from a common disadvantage - low energy efficiency due to heavy heat losses.
As shown by the analysis of heat losses, most of the heat is lost in the contact zone of a slag bath and the surface of molding devices.
In one of the present-art methods of effecting an electroslag remelting process heat losses are decreased by providing an electrode being melted with a protective shield. With the above method the consumable electrode is dipped into a slag bath, as it is being fused under the effect of electric current flowing through the electrode and slag, said slag bath accommodating also a protective shield made as a cylindrical ring encompassing the electrode part immersed in the slag bath.
The lower end of such a shield is constantly sustained on the level of the lower end of the consumable electrode.
A plant for effecting said method comprises a protective shield made as a cylindrical ring encompassing a consumable electrode. The protective shield is cantilevered on a vertical transfer gear, the bottom end of this protective shield being always maintained on the level of the lower end of the consumable electrode with the aid of said gear and a device for monitoring the required depth of immersion of the protective shield in the slag bath.
The known method of effecting an electroslag remelting process, in which the protective shield is carried by means of the vertical transfer gear calls for sophisticated equipment, such as, a shield transfer gear and a device for monitoring the requisite depth of immersion of the shield in the slag bath.
This known method fails to control current distribution in a slag bath, to decrease thermal radiation losses of the slag bath and to obviate the oxidation of easily-oxidizable elements contained in metal.
The main object of the present invention is to provide a method of effecting electroslag remelting processes in which all the heat liberated due to the passage of electric current through an electrode and a slag bath would be essentially employed for melting the electrode and heating the molten slag bath.
These and other objects of the present invention are achieved by the fact that in a method for effecting an electroslag remelting process, comprising the steps of shielding an electrode, dipping it into a slag bath, passing electric current through the electrode and slag bath, said current heating the slag to at least a metal melting point, according to the invention, shielding is effected upon producing molten slag and establishing a metal pool by lowering on the surface of the slag bath of a disc with an opening for the passage of the electrode so that the disc will float in the slag without coming in contact with the metal pool, the disc floating in the slag bath since it is manufactured from material with a specific density lower than that of the slag.
By carrying out the electroslag remelting process in the above manner it is possible to diminish heat losses in the zone of contact of a slag bath with the surface of a molding device, e.g., a mold. It enables also a nearly complete reduction in thermal radiation losses of the slag bath and redistribution of current densities in the melting zone of the consumable electrode. All these factors provide for a higher electrical energy efficiency.
It is expedient that with the above method of effecting an electroslag remelting process a disc of a material superior in electrical conductivity to molten slag is lowered on the surface of a slag bath.
In this way it is possible to provide current concentration around the melting electrode by compressing an electric field.
While using the above method it is good practice if a disc lowered on the surface of a slag bath is produced from a material superior in its deoxidizing ability to elements contained in the metal and slag.
This would make it possible to preserve such easily-oxidizable elements as Al, Ti, Si and others in the metal being remelted. In this case a better effect is attained if the disc descended on the bath surface is maufactured from material forming during deoxidation reactions which generate gaseous products that are removed together with evolving gases.
It is expedient that the disc lowered on the surface of the slag bath be preheated, its heating temperature being selected within a frange from about 500.degree. C. to about 1200.degree. C. depending on technological requirements.
Usually the disc is produced from graphite, grahitic carbon or other carbonaceous material.
Therefore for producing protective atmosphere which would protect the molten slag and melting electrode against oxidation the disc must be preheated to a temperature of at least +500.degree. C.
This stems from the fact that carbon contained in the disc combines with oxygen from ambient air, forming the protective atmosphere, at a minimum temperature of +500.degree. C.
We have found that +1200.degree. C. must be considered the most favorable preheating temperature, since it can be attained by using conventional heating sources, and since the introduction of a disc, heated to such a temperature, into a slag bath does not distrub the stability of electroslag remelting, an average disc temperature during the electroslag remelting process approximately +1200.degree. C.
Thus, the preheated disc does not disturb the stability of the process at the moment it is being fed into the slag bath, and it provides the creation of a protective atmosphere at such moment.
As the disc is consumed, similar discs are generally lowered thereon from above.
This technique must be resorted to when effecting a continuous electroslag remelting process, when a disc lowered initially on the surface of a slag bath is consumed before the completion of the refining process.
In accordance with these and other objects, the essence of the present invention consists also in that in a device for effecting an electroslag remelting process, comprising a baseplate mounting a means for carrying out an electroslag remelting process, accommodating a molten slag bath with an electrode that is provided with a protective shield and connected to a current source, according to the invention, the protective shield is made as a disc of a material whose specific density is lower than that of the slag, said disc overlapping essentially the entire surface area of the slag bath and being fitted with an opening for the passage of the electrode and arranged in the means for effecting an electroslag remelting process with a clearance between the internal surface of this means and a disc side wall, amounting to at least the thickness of a scull crust on the means interior, its clearance between the wall of the disc opening and the electrode being equal to at least the thickness of a scull crust on the electrode.
Since the protective shield is made as a disc in material whose specific density is lower than that of the slag, it will float on the slag surface functioning as a protective shield overlapping essentially the entire surface area of the slag bath.
Due to the opening provided in the disc for the electrode passage and forming the clearance with the electrode, as well as due to the clearance between the internal surface of the means for effecting an electroslag remelting process and the disc side wall, the disc can freely move upwards as the metal is being built up. To prevent the disc from being stuck as it moves upwards the above clearances must be equal to at least the thickness of a scull crust formed on the internal surface of the means for effecting an electroslag remelting process and on the electrode.
The protective shield can be made as a disc of increased or increasing thickness towards its periphery.
Such discs are advisable to be used when it is necessary to redistribute thermal power of the slag bath from its centre to the periphery.
It is also expedient that the protective shield be made as a disc of increased or increasing thickness towards its center.
In this case the heat of the slag bath will be concentrated in its central portion.
It is sound practice that the disc be provided with through conduits running inside it from the center to the periphery.
This would provide an intesne stirring of the slag and better deoxidation during the electroslag remelting process.